1. Field of the Invention
The present invention relates to an image processing apparatus that converts input image data into halftone image data, an image forming apparatus having the image processing apparatus, a program, and a method of image processing.
2. Description of the Related Art
In the case of halftone representation in image forming apparatus of electrophotographic system, a halftone processing is carried out using a threshold matrix in which threshold values are arranged substantially radially so as to be larger from the center of the threshold matrix to obtain a dot-like shape output that is similar to an image printed by a conventional printer (for example, see Patent Document 1).
An example of the threshold matrix used for representing 256 gradations with 9 pixels is shown in FIG. 9. The threshold matrix in FIG. 9 is provided with elements 1 to 9 that are respectively correlated to, for example, threshold LUTs (Look Up Tables) shown in FIG. 10, and when the halftone processing such as a dither processing is applied to image data using this threshold matrix, dots are generated in the order of the element number from a position of a pixel corresponding to an element 1, so that the dot-like shape (eg. circular shape) can be maintained regardless of gradation differences.
[Patent Document 1] Japanese Patent Publication Laid-Open No. 2004-80109
When forming an image in the image forming apparatus of the electrophotographic system, the output characteristic for one pixel has an S-like shape as shown in FIG. 11 reflecting the rise-time in the laser emission. Under such an output characteristic, dots are generated in the order of the element number shown in FIG. 9, wherein the rise-time of an output signal is delayed each time the dot is generated at a position in which the pixels disposed in the sequential order of the dot generation are not adjacent side by side, and a gradation jump occurs, thereby the gradation property of the input image is deteriorated. In order to prevent this, adjustment has been carried out, such as, that applies a γ curve to every gradation characteristic in the dot generation to modify the output characteristic of FIG. 11 as linear as possible. However, because the gradation characteristic in the rise-time is influenced by the dot generation order, this method has needed substantial amount of adjustment work in order to obtain the gradation without jump.